SIr John Chilcot has again defied calls to set a timetable for publication of the Iraq Inquiry report - insisting he must make sure it is "fair".
The inquiry chairman said he understood the "anguish" of families who lost loved ones in the conflict, but argued that the probe was "unprecedented" in its scope.
He also defended the controversial Maxwellisation process, which means the inquiry seeks responses from everyone facing criticism before its conclusions are published.
The statement came amid renewed pressure to explain why the report has yet to emerge, six years after it was first commissioned by former Prime Minister Gordon Brown.
However, Sir John's justification was immediately rejected by bereaved families, who have been threatening legal action to force him to publish.
"I should like firstly to reiterate that my colleagues and I understand the anguish of the families of those who lost their lives in the conflict," Sir John said.
"We take the responsibility we were given as an independent inquiry extremely seriously, and understand the need for Government, Parliament and the public to see our report as soon as possible."
But added: "It is critically important that the report should be fair to all who participated in the conflict and to those who bore the responsibility of taking decisions ...
"In its scope and length, this is an inquiry mandate for which there is no precedent.
"Given the scale of the task of assembling a reliable account of a nine-year period and drawing conclusions on a wide range of issues, it became apparent as the work proceeded that the report would have to be very long and would take a considerable time to produce."
Sir John said Maxwellisation was "essential" to ensure that "conclusions drawn by the inquiry are robust and that any criticism included in the final report is soundly based, fair and reasonable".
"Individuals have not been given an open-ended timescale and Maxwellisation is not a process of negotiation. The inquiry has remained in control of its deadlines throughout the process. In some cases, the response sent to us required detailed and complex analysis which has taken time," he said.
"The Maxwellisation process is essential not only to the fairness but also the accuracy and completeness of our report.
"It has already led, for example, to the identification of government documents which had not been submitted to the inquiry and which have in some cases opened up new issues."
Sir John said the inquiry expected to receive the last Maxwellisation responses "shortly" and would then be able to produce a timetable for the report.
"That will allow us to complete our consideration of the responses, to decide what further work will be needed, and to provide the Prime Minister and thus Parliament and the public with a timetable for the publication of our work," he said.
He also revealed that the inquiry had responded to lawyers acting for some families, who had threatened to launch legal action if a timetable had not been set for publication of the report by 5pm today - although he declined to say what was in the response.
Roger Bacon, whose son Major Matthew Bacon was killed in Iraq in 2005, said he was "disappointed" Sir John had not revealed when he will publish his report and accused him of failing to understand the families' anguish.
Mr Bacon told the Press Association: "He should just get on with it. I'm not sure he is able to understand our anguish. If he was in our shoes, he might well take a different view on what is going on.
"This process of Maxwellisation is just too much. It seems to go on and on and on. The fact he is still waiting for responses means there will be further delays."
Reg Keys, whose son Lance Corporal Tom Keys was killed in Iraq in 2003, said the families wanted the report to be "open, transparent and unflinching in its findings".
He told Sky News: "The letter to Sir John clearly stated that unless we get a timeframe for completion of this report which was the end of this calendar year then we will be looking down the avenue of taking legal action against Sir John because we feel now that the delay over this Maxwellisation, which has caused the delay in the final report.
"He has had adequate funding, that delay is borderline unlawful, so we will seek a judicial review to see if the courts can press Sir John under the legal channels for a timeframe to complete before the end of this year."
Rose Gentle, whose 19-year-old son Gordon was killed in a bomb attack in 2004, said: "Chilcot says he understands the anguish of the families, but he's not the one going to bed and having nightmares, dreaming about it every night."
Clare Short, Labour's international development secretary at the time of the war, said she did not believe the report was being held up by the Maxwellisation process.
She said all of those criticised had to reply to the inquiry within a deadline of a few weeks, which passed "a long time ago", and claimed Sir John was probably having to redraft the report as the current version is "very poor".